Filter

ABSTRACT

A rigid filter, useful for example for filtering molten thermoplastic resin as it is forced through a spinnerette head, comprising a rigid block of shaped uniformly permeable material, having continuity and identity of composition and having increased filtering capacity, with a plurality of filtration cavities extending into the block from the top surface and a plurality of drainage cavities extending into the block from the bottom surface.

ttes 1 ergstrom "Ht. mm mm NM 8 wwD m BHM 950 337 999 NH 578 PrimaryExaminer-Samih N. Zaharna Assistant Examiner-T. A. Granger .mam

MMM

Attorney, Agent, or FirmHarold J. Kinney et a1.

22 Filed: Sept. 30, 1971 [57] ABSTRACT A rigid filter, useful forexample for filterin thermoplastic resin as it is forced throu Appl.No.: 185,270

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532,939 1/1895 3,581,902 6/1971 Bidler FILTER This invention relates tofilters made from consolidated particulate materials, such as sinte redpowdered metal or ceramics or glass beads.

In the spinning of molten thermoplastics to form fibers, it is known toplace in the spinnerette head assembly a filter to remove solidparticles and other inhomogeneities. Various filters have been, proposedor used for this purpose, such as a body of sand (e.g., see U.S. Pat.No. 2,607,954, issued Aug. 26, 1952 to G. Schneider et al) or a filtermade of metal plates with bore holes (e.g., see U.S. Pat. No. 2,792,122,issued May 14, 1957 to W. Munch' et al or a filter made of porous metal(e.g., see U.S. Pat. No. 3,295,161, issued Jan. 3, 1967 to L. H. Mott).Filters made from sintered powdered metal using powder metallurgytechniques have also been disclosed (see, for example, U. S. Pat. No.3,570,059, issued March 16, 1971 to L. H. Mott). Though such filtershave enjoyed some use, they inherently have a number of disadvantages.The prior art powdered metal filters generally require relatively thickelements to obtain needed strength at a sacrifice of flow rate orthroughput, or require mechanical fastening of porous metal parts toeach other. Some of the prior art filters, particularly those designedto have extended area filtering surface, have been found to be generallystructurally weak and fragile at the thicknesses and micronic ratingsrequired. This defect causes such filter structures to collapse in useunder high pressure filtration. To overcome this weakness, necessarysupport members, such as support plates or breaker plates and screens,have been incorporated into the filter support structure, but theaddition of supporting elements increases the cost of the filterstructures and adds nothing to the filtering capabilities.

The filter of this invention is a physically rigid, uniformly permeablestructure, having an extended area filtering surface, which does nothave shortcomings of the prior art filters. Briefly, the filter of thisinvention is a shaped block of uniformly permeable material with top andbottom surfaces, the top surfacehaving a plurality of filtrationcavities extending into the body of the permeable block and providing anextended area filtering surface and the bottom surface having aplurality of drainage cavities extending into the porous block.

In the accompanying drawing,

FIG. 1 is a plan top view of one embodiment of the filter of thisinvention.

FIG. 2 is a sectional view of .the filter of FIG. 1 taken along theplane 2-2.

FIG. 3 is a longitudinal section in partial section of a spinnerettehead incorporating the filter of FIGS. 1 and 2, In the various figures,like reference numbers have been used to designate like parts.

Referring now to the accompanying drawing and initially to FIGS. 1 and2, which show a preferred structure of this .invention, reference number1 generally designates acylindrical block of rigid permeable materialhaving a top surface 2 with a plurality of blind cylindrical filtrationcavities 3'depending therefrom, said top surface and the surfacesdefining said cavities providing an extended filter area or surfaceadapted to be exposed to the material to be filtered. The opposed lowersurface 4 has a plurality of blind cylindrical tenitic stainless steel.

drainage cavities 6 extending upwardly into the block.

The filtration cavities 3 and drainage cavities 6 of FIG. 1 are inparallel alignment and spaced apart in a hexagonal pattern. In thepreferred embodiment shown in the drawing, each filtration cavityadjacent the outer wall 5 or periphery of the block has at least fourequally spaced drainage cavities surrounding it and the nonperipheralfiltration cavities have six equally spaced drainage cavitiessurrounding it to provide ready drainage from the filtration cavities.This configuration is the desirable pattern for the closest spacing ofthe cavities within a given volume while maintaining a distance betweencavities sufficient to retain the integrity and strength of the filterduringuse. The close packing of filtration cavities 3 together withtheir depending geometry provides a maximum extended area filteringsurface for a given thickness of filter. The cavities need not be formedas cylinders nor arranged in a hexagonal manner, and other suitableshapes of blind cavities and special arrangements will be apparent toeffectuate desirable filtering and strength characteristics of thefilter.

FIG. 3 shows a spinnerette head assembly having a feed duct 7 which'cancommunicate with a suitable source of molten material to be supplied forfiltering at a controlled rate and under high pressure. The filter ofFIG. 1 is shown disposed within the filtration cavity 12 of thespinnerette retainer 10. The filter 1 can be separated from thespinnerette disk 9 by a void 8 to allow lateral flow of the filteredmolten material to the spinnerette extrusion orifices l1.

In operation, the material to be' filtered, such as a moltenthermoplastic (e.g., polyester) at extrusion pressures as high asseveral thousand pounds per square inch, is fed through the feed duct 7into the spinnerette head assembly. The molten plastic is forced againstthe extended area filtering surface presented by the top 2 and blindfiltration cavities 3 of filter l and flows through the permeable wallsthereof. The filtered material enters the drainage cavities 6, passesthrough the void 8 (or a screen, not shown) and is extruded throughextrusion orifices 11 in the spinnerette head 9. The pressure on thefilter 1 will be distributed to the step of the retainer 10. Where theouter vertical wall 5 of the filter is made with a downward and inwardtaper, the downward force on the filter block will be almost entirelytransmitted to the spinnerette retainer 10.

The preferred materials for fabricating the filter of this invention arepowdered metals, particularly aus- Austenitic chromium-nickel stainlesssteels generally contain 16.0 to 26.0 weight percent chromium, 6.0 to22.0 weight percent nickel, 0.03 to 0.25 (max) weight percent carbon andoccasionally some other elements added to develop certain specificproperties, such as 1.75 to 4.00 weight percent molybdenum or smallamounts of titanium, tantalum, and niobium to minimize formation ofchromium carbides. Standard types of these steels have been assignednumbers and specifications by the American Iron and Steel Institute.These are generally known in the art as stainless steels of the AISI 300Series. All of these AISI steels of the 300 Series are applicable in thepractice of this invention. Of course, other powdered metals can be usedin fabricating filters of this invention, such as nickel, tungsten,copper and the like, and alloys of such metals, including bronze, monel,etc. Particularly sintered ceramic materials or bonded glass beads canalso be formed into a porous block and used in this invention.

The preferred filters are made from powdered metal which is relativelycoarse so that upon sintering, the resulting shaped article will havethe desired permeability or micronic rating. Powdered metal with meshsizes in the range of 20+325 can be used, such as 200+325, l+200,50+l00, -20+50, or blends thereof, can be selected to produce thedesired micronic rating or bubble point, and to that end small amounts,e.g., 1-20 weight percent, of 325 mesh can be blended with the coursepowder, with the 50+325 mesh. (The term mesh" referred to herein meansmesh size according to US. Standard Sieve and bubble point a measure ofthe air pressure on one surface of a filter required to produce thefirst air bubble on the submerged opposite surface, corresponds to theabsolute micronic rating of the filter as described in WADC T.R. 56-249and MILF88 1 5B). The use of powdered metal with these mesh ranges willenable one to make filter structures in accordance with this inventionwith various micronic ratings, e.g., in the range of l-l50 microns. Forpurposes of making a filter having a very low micronic rating, e.g.,less than microns, powdered metal having a mesh size of 325 can be used.

In fabricating the filter from powdered metals, powdered metal of thedesired mesh is blended with an organic heat-fugitive binder, such asthose disclosed in US. Pat. Nos. 2,592,943, 2,709,651, and 2,902,363;the presently preferred binder is methyl cellulose in combination with athermosetting epoxy. Solvents may be used in conjunction with thesebinders, such as water, as well as plasticizers, such as glycerin. Theblending can be carried out in a conventional manner in various types ofcommercially available mixers, blenders, tumblers, and the like, carebeing taken to insure that the blend is homogeneous and the componentsare well dispersed. The resulting blend will be in the nature of aplastic mass or dough and will be similar in consistency to that ofmodeling clay. The plastic mass can be shaped in a suitable mold to thedesired block configuration with the internal cavities and then dried toform a green structure, or the mass can be formed into a block and driedto form a green structure which is then shaped to the desiredconfiguration by drilling filtration and drainage cavities and turningthe block to the final shape or the use of other normal machiningprocedures.

The formed green structure is then heated in an oxygen-containingatmosphere to burn off the binder and then, the green structure issintered under vacuum or a suitableatmosphere, such as a reducingatmosphere like hydrogen or dissociated ammonia. Sintering atmosphere,temperature, and the duration of sintering will depend upon theparticular powdered metals being used, the selection of these conditionsbeing within the skill of the art. In the case of austenitic stainlesssteels mentioned above, a hydrogen or dissociated ammonia atmospherewith a dew-point of -40 F. or lower and sintering temperatures in therange of l200 to l400 C., preferably 1250 to 1350 C., will be suitable,and the duration of sintering will usually be from 10 minutes to two orthree hours.

The filter structure ofthis invention is entirely made in one piece ofconsolidated particulate materials without requiring wrought metalcomponents. The filter has no press-fitted parts that could be dislodgednor are there any exposed protuberances which are subject to deformationor failure at high pressure differential filtration. An importantadvantage or feature of the filter of this invention is its rigidintegral structure which enables the filter to withstand pressuredifferential stresses, and other forces normally encountered duringoperation, without being prone to the deformation, rupture, separationor other types of failure experienced with prior art filters which donot have the inherent features of this invention. This filterconfiguration eliminates the need for the breaker plate assemblynormally required in filtering molten thermoplastic materials. Theabsence of a breaker plate reduces the dwell time of the molten polymerin the filtration cavity, thereby reducing the possibility of thermaldegradation and gel formation. In spinnerette heads without a supportingstructure, the height of the filter may be increased allowing deeperblind filtration cavities, resulting in greater filtration capacity dueto the increase in the expanded surface of a deeper filter. Thisconstruction also will withstand reverse flow conditions andback-flushing cleaning operation without the likelihood of failure.Higher operating pressures can be used without fear that the filterstructure will rupture or be deformed. It is generally desirable whenoperating under high pressure conditions to have a minimum wallthickness of H16 inch between the filtration cavities and the drainagecavities to prevent a failure of the filters due to collapse.

The unique structure of this filter allows the filtration of moltenthermoplastic material such as polyester resins. The filter structure ofthis invention will produce high quality filtered material suitable forapplication in magnetic tape or synthetic fiber which require a filteredmaterial with a high degree ofuniformity and free of unmelted solids.

The objects and advantages of this invention are further illustrated bythe following examples, but particu lar materials and amounts recited inthis example, as well as other conditions and details, should not beconstrued to unduly limit this invention.

EXAMPLE 1 An extended surface area filter similar to that shown in FIG.1 was prepared by first mixing 1425 grams of lO0+200 mesh austeniticchromium-nickel stainless steel powder (MI 3 16L), grams molybdenum(Fisher No. of about four microns), 49.6 grams of 4000 cps.methylcellulose (Methocel). The resulting mixture was blended in a sigmablade double arm mixer with 55 grams ofWaterpoxy M205 (a liquidbispheno] A epichlorohydrin based epoxy resin described in General MillsBulletin A23822A of May 1, 1970) 1.37 grams nickel chloride imidazolecatalyst, 250 grams of water and 70 grams of glycerin to form a plasticmass.

The plastic mass was formed into a number of right cylinder blocks in adie, using a simple plunger and 2% inch die cavity. The die was heatedto about C. and the plastic mass charged into the heated die. A pressureof approximately 2000 pounds per square inch was applied to the plunger,and the pressure and temperature were held approximately three minutes.The cylinder blocks were then ejected from the die and dried in an aircirculating oven at F. for 30 hours. After the drying, resultingcylinder blocks were approximately 1% inch in height.

After drying, the green cylinder blocks of green material were very hardand amenable to machining. The green blocks were turned in a lathe toproduce true right cylinders and then drilled with non-connecting blindholes forming l9 filtration cavities A inch in diameter and inch deepand '36 blind drainage cavities 3/32 inches in diameter and 4 inch deepin the porous cylinder. The filtration cavities were drilled in ahexagonal pattern on 0.362 inch centers and the blind drainage cavitieswere drilled on vertical axis equidistant from the vertical axes of thethree nearest filtration cavities. The blocks were heated, inan airatmosphere, to 700 F. in four hours and held at 700 F. for three hoursto remove the binder. The shaped green cylinder blocks were thensintered in a hydrogen atmosphere with a dew point of 40 F. for twohours at 1350 C.

The resulting extended surface area filters were each approximately oneinch in height and 2 A inches in diameter. The bubble points of theextended surface filters gave absolute micronic ratings ranging from 60to 70 microns.

This filter showed its superior rigidity to high pressure filtrationwhen compared with a metallurgically integral filter having a pluralityof protruberances and similar in design to that disclosed in co-pendingapplication, Ser. No. 135,638, filed April 20, 1971. The filter withprotuberances failed at a hydrostatic pressure of approximately 500pounds per square inch. The filter of this invention prepared asdescribed above, having substantially the same extended filtration area,showed no deflection at pressures-in excess of 1500 pounds per sauareinch, and showed little deflection even at 4150 pounds per square inchof hydrostatic pressure at which point the hydrostatic test wasterminated without failure.

EXAMPLE 2 Glass beads having an average particle diameter of 470 micronswere coated with an epoxy resin (Scotchkote l 12) according to themethod disclosed in U.S. Pat. No. 3,175,935, issued March 30, 1965 to R.C. Vanstrum. The resin-coated beads were poured into a 2 /2 inchdiameter cylindrical pressing die which had been coated with peanut oilas a mold release agent. A pressure of 4000 psi and temperature of 160F. was applied to the glass beads for several minutes resulting in a 2 Ainch diameter right cylinder two inches high.

The right cylinders were transferred to an air oven and heated to 375 F.for one-half hour to cure the epoxy. The firmly bonded permeable rigidstructure was machined in accordance with Example 1, the resultingextended surface filter having a micronic rating of ap proximately 1microns.

EXAMPLE 3 A filter was made using glass beads with an average diameterof 200 microns, following the procedure of Example 2. A micronic ratingof approximately 56 microns was obtained for the resulting filter.

EXAMPLE 4 An extended surface area filter was prepared by mixing 300grams 28+48 mesh aluminum (Alcoa T-6l 700 grams of Alcoa A-2" alumina,and 70 grams TABLE 1 Operation Time (hours) Room temperature to 200 C.

I hold 200 C. 1 200 to 400 C. 2 400 to 500 C. 3 500 to 950 C. 3 950 to1000 C. 1% I000 to l200 C. 10 1200 to 1550 C. 1 hold 1550 C. 6%

Various modifications and alternations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth herein. What is claimed is:

1. A unitary, rigid filter structure comprising a permeable block ofconsolidated, particulate material free of protuberances and having atop surface with a plurality of blind filtration cavities verticallyextending into the block from inlet openings located wholly in said topsurface, and an opposing surface with a plurality of blind drainagecavities vertically extending into the block from outlet openingslocated wholly in said opposing surface, said cavities being spacedapart from one another and spaced from the vertical outer periphery ofsaid block.

2. The filter according to claim 1 wherein said porous materialcomprises sintered powdered stainless steel.

3. The filter according to claim'l wherein said particulate materialcomprises epoxy bonded glass heads.

4. The filter according to claim 1 wherein said particulate materialcomprises sintered powdered ceramic material.

5. A unitary, uniformly permeable filter structure comprising acylindrical, permeable block of consolidated, sintered powdered metalwith a base portion having a cross-sectional area greater than onesquare inch in minimum dimension, said base having a plurality of blinddrainage cavities vertically extending into the block from outletopenings located wholly in the bottom surface of said block, and havingan opposed extended area filtration surface defined by a plurality ofblind filtration cavities vertically extending into the block from inletopenings located wholly in the top surface of said block, said cavitiesbeing spaced apart from one another and spaced from the vertical outerperiphery of said block, said block being free of protuberances.

6. A filtration assembly comprising in combination: a housing having aninlet and outlet with a filter chamber disposed within said housing, aunitary, rigid, filter structure disposed within said filter chamber,said filter structure comprising a permeable block of consoliand spacedfrom the vertical outer periphery of said block.

7. A unitary, rigid filter structure comprising a permeable block ofconsolidated, particulate material free of protuberances and having atop surface with a plurality of blind filtration cylindrical chambersvertically extending therefrom into the block and an opposing surfacewith a plurality of blind drainage cylindrical chambers verticallyextending therefrom into the block.

1. A unitary, rigid filter structure comprising a permeable block ofconsolidated, particulate material free of protuberances and having atop surface with a plurality of blind filtration cavities verticallyextending into the block from inlet openings located wholly in said topsurface, and an opposing surface with a plurality of blind drainagecavities vertically extending into the block from outlet openingslocated wholly in said opposing surface, said cavities being spacedapart from one another and spaced from the vertical outer periphery ofsaid block.
 2. The filter according to claim 1 wherein said porousmaterial comprises sintered powdered stainless steel.
 3. The filteraccording to claim 1 wherein said particulate material comprises epoxybonded glass beads.
 4. The filter according to claim 1 wherein saidparticulate material comprises sintered powdered ceramic material.
 5. Aunitary, uniformly permeable filter structure comprising a cylindrical,permeable block of consolidated, sintered powdered metal with a baseportion having a cross-sectional area greater than one square inch inminimum dimension, said base having a plurality of blind drainagecavities vertically extending into the block from outlet openingslocated wholly in the bottom surface of said block, and having anopposed extended area filtration surface defined by a plurality of blindfiltration cavities vertically extending into the block from inletopenings located wholly in the top surface of said block, said cavitiesbeing spaced apart from one another and spaced from the vertical outerperiphery of said block, said block being free of protuberances.
 6. Afiltration assembly comprising in combination: a housing having an inletand outlet with a filter chamber disposed within said housing, aunitary, rigid, filter structure disposed within said filter chamber,said filter structure comprising a permeable block of consolidated,particulate material having a top surface with a plurality of blindfiltration cavities extending into the block from inlet openings locatedwholly in said top surface, and an opposing surface with a plurality ofblind drainage cavities extending in the block from outlet openingslocated wholly in said opposing surface, said filtration cavities beingin communication with said inlet via said inlet openings and saiddrainage cavities being in communication with said outlet via saidoutlet openings, said block being free of protuberances, and saidcavities being spaced apart from one another and spaced from thevertical outer periphery of said block.
 7. A unitary, rigid filterstructure comprising a permeable block of consolidated, particulatematerial free of protuberances and having a top surface with a pluralityof blind filtration cylindrical chambers vertically extending therefrominto the block and an opposing surface with a plurality of blinddrainage cylindrical chambers vertically extending therefrom into theblock.